Mass-radius diagram for transiting hot Jupiters (grey circles; P_orb < 10 days and 0.1 <M_p< 15M_Jup, from the Extrasolar Planet Encyclopedia at http://exoplanet.eu/, as of 15 September 2014). Kepler-423b is marked with a thicker blue circle. The Fortney et al. (2007) isochrones for a planet core mass of 0, 25, 50 M_⊕ – interpolated to the solar equivalent semi-major axis of Kepler-423b and extrapolated to an age of 11 Gyr – are overplotted with thick green lines from top to bottom. Isodensity lines for density ρ_p = 0.25, 0.5, 1, 2, and 4  g cm^-3are overlaid with dashed lines from left to right.